Studies concerning the neural control of sexual behavior in the rat revealed that: (1) Cholinergic stimulation of the preoptic-septal region facilitated sexual receptivity. (2) Intracerebral implants of cyclic AMP failed to mimic the facilitatory influence of progesterone. (3) Intracerebral implants of estradial induced male sexual behavior when positioned in the preoptic area but not in the posterior hypothalamus. Experimentation on the sex differentiation of behavior revealed: (1) Female rats fetuses which develop next to a male fetus are virilized morphologically and behaviorally; (2) Pharmacological blockade of testosterone metabolism reduces its masculinizing effects on behavior of hamsters neonatal development; (3) The anti-estrogen MER 25 also antagonized the masculinizing effects of testosterone upon behavior. The latter two findings suggest that the masculinizing effects of testosterone upon behavior may be the result of an estrogen metabolite.